Allow unions to be read when there is no matching case or default for the specified discriminator value

packages/omgidl-serialization/src/MessageReader.test.ts

@@ -752,6 +752,87 @@ module builtin_interfaces {
       },
     });
   });
+
+  it("Reads mutable union with default case with id where discriminator case does not exist", () => {
+    const msgDef = `
+        @mutable
+        union ColorOrGray switch (uint8) {
+          case 0:
+            @id(100)
+            uint8 rgb[3];
+          default:
+            @id(200)
+            uint8 gray;
+        };
+        @mutable
+        struct Fence {
+            @id(5) ColorOrGray color;
+        };
+    `;
+
+    const writer = new CdrWriter({ kind: EncapsulationKind.PL_CDR_LE });
+    writer.emHeader(true, 5, 6); // writes emHeader for color field
+
+    writer.emHeader(true, 1, 1); // emHeader for discriminator (switch type)
+    writer.uint8(0x09); // then writes uint8 case for gray
+
+    writer.emHeader(true, 200, 1); // emHeader for field (gray)
+    writer.uint8(55); // then writes uint8
+
+    writer.sentinelHeader(); // end union
+    writer.sentinelHeader(); // end struct
+
+    const rootDef = "Fence";
+    const reader = new MessageReader(rootDef, parseIDL(msgDef));
+
+    const msgout = reader.readMessage(writer.data);
+
+    expect(msgout).toEqual({
+      color: {
+        [UNION_DISCRIMINATOR_PROPERTY_KEY]: 9,
+        gray: 55,
+      },
+    });
+  });
+  it("Reads mutable union field with with id where discriminator case does not exist and there is no default", () => {
+    const msgDef = `
+        @mutable
+        union ColorOrGray switch (uint8) {
+          case 0:
+            @id(100)
+            uint8 rgb[3];
+          case 3:
+            @id(200)
+            uint8 gray;
+        };
+        @mutable
+        struct Fence {
+            @id(5) ColorOrGray color;
+        };
+    `;
+
+    const writer = new CdrWriter({ kind: EncapsulationKind.PL_CDR_LE });
+    writer.emHeader(true, 5, 6); // writes emHeader for color field
+
+    writer.emHeader(true, 1, 1); // emHeader for discriminator (switch type)
+    writer.uint8(0x09); // then writes uint8 case for gray
+
+    // absent value because discriminator doesn't exist
+
+    writer.sentinelHeader(); // end union
+    writer.sentinelHeader(); // end struct
+
+    const rootDef = "Fence";
+    const reader = new MessageReader(rootDef, parseIDL(msgDef));
+
+    const msgout = reader.readMessage(writer.data);
+
+    expect(msgout).toEqual({
+      color: {
+        [UNION_DISCRIMINATOR_PROPERTY_KEY]: 9,
+      },
+    });
+  });
   it("Reads array from mutable union field", () => {
     const msgDef = `
         @mutable

packages/omgidl-serialization/src/MessageReader.ts

@@ -119,17 +119,24 @@ export class MessageReader<T = unknown> {
     }
     const discriminatorValue = deserInfo.switchTypeDeser(reader) as number | boolean;
 
+    // Discriminator case determination: Section 7.4.1.4.4.4.2 of https://www.omg.org/spec/IDL/4.2/PDF
     // get case for switchtype value based on matching predicate
-    const caseDefType = getCaseForDiscriminator(deserInfo.definition, discriminatorValue);
+    let caseDefType = getCaseForDiscriminator(deserInfo.definition, discriminatorValue);
+    // If no case is found, use the default case
     if (!caseDefType) {
-      throw new Error(
-        `No matching case found in ${
-          deserInfo.definition.name ?? ""
-        } for discriminator value ${discriminatorValue.toString()}`,
-      );
+      caseDefType = deserInfo.definition.defaultCase;
     }
 
-    const fieldDeserInfo = this.deserializationInfoCache.buildFieldDeserInfo(caseDefType);
+    const fieldDeserInfo = caseDefType
+      ? this.deserializationInfoCache.buildFieldDeserInfo(caseDefType)
+      : undefined;
+
+    // if no matching case and no default case, only return discriminator value
+    if (!fieldDeserInfo || !caseDefType) {
+      return {
+        [UNION_DISCRIMINATOR_PROPERTY_KEY]: discriminatorValue,
+      };
+    }
 
     return {
       [UNION_DISCRIMINATOR_PROPERTY_KEY]: discriminatorValue,